Oscillators are electrical devices that generate an oscillating or repetitive, signal. The signal comprises a voltage which varies in magnitude and sign over time. The signal can be a sinusoidal wave, such as in an analog signal, or a square wave, such as in a digital or electronic signal. Signals generated by an oscillator, especially electronic signals, have a number of applications such as, for example, a precise reference clock source, in a voltage-controlled oscillator for frequency tuning, or in a phase-locked loop for locking onto another signal.
A common type of oscillator is an LC oscillator. An LC oscillator consists of an inductor (L) and a capacitor (C) connected in parallel or in series to form a resonator. In this arrangement, electrical charge flows back and forth, as a harmonic oscillation (oscillations), between the plates of the capacitor through the inductor. LC oscillators generally, and LC oscillators in integrated circuits specifically, do not have a high quality factor. For example, LC oscillators in an integrated circuit have a quality factor between 5 and 25. The quality factor of an oscillator describes how under-damped its resonator is—the higher the quality factor, the lower the rate of energy loss relative to the stored energy of the oscillator. A loaded quality factor is the quality factor a resonator when “loaded” or combined with additional losses due to the circuitry attached thereto. Because LC oscillators have a low quality factor, oscillations therein quickly die out unless powered by another source.
Electro-mechanical resonators are devices which can be employed within oscillators to control the frequency and improve the quality of oscillations. This, accordingly, improves the phase purity of the signal produced by the oscillator. A piezoelectric resonator is a type of electro-mechanical resonator. In a piezoelectric resonator, a voltage or charge applied to the resonator generates a mechanical strain in the resonator. Conversely, a mechanical strain applied to the resonator causes the resonator to generate a charge and voltage difference across its terminals. When the resonator forms part of a circuit, energy in the resonator oscillates between mechanical energy and electrical energy causing the resonator to physically vibrate during oscillation. Resonators inherently resonate at particular frequencies or their resonance frequencies. The physical dimensions of the resonator determine its resonance frequencies. For example, in bulk-acoustic wave (BAW) resonators, the thickness determines the wave length and hence the frequency.
A common resonator is a quartz crystal. Oscillators which use this type of resonator are known as crystal oscillators (XOs). Quartz resonators operate at resonance frequencies ranging from tens of kilohertz (kHz) to tens of megahertz (MHz), with the overtone (harmonic) oscillations being up to a few hundreds of MHz. Although a thinner quartz resonator creates oscillations with a shorter wavelength and a higher frequency, there is a limit to how thin a quartz resonator can be cut. Accordingly, there is also a limit on the maximum achievable resonance frequency produced in an oscillator using a quartz resonator.
Other types of piezoelectric resonators include, but are not limited to, film bulk acoustic resonator (FBAR) resonators and bulk acoustic wave (BAW) resonators. These types of resonators can be made micrometers thick and, accordingly, can operate in the gigahertz (GHz) resonance frequency range. They can also be used within oscillators to provide a high quality factor, and can handle high power. A BAW resonator, for example, comprises a thin layer of piezoelectric material such as aluminum nitride (AlN). On each side of the material is a metal electrode for conducting a current or voltage to the material. The quality factor of an FBAR/BAW resonator is between 500 and 3000 when operating in the 1-7 GHz range. This is significantly greater than the quality factor of oscillators using integrated LC resonators, which is approximately between 5 and 25 when operating in the same multi-gigahertz frequency range. Another type of piezoelectric resonator is a surface acoustic wave (SAW) resonator.
Quartz crystal and SAW resonators as well as FBAR/BAW resonators require active drive circuitry to initially excite, and then maintain, the resonator's oscillations. Oscillators comprise this active drive circuitry. The active drive circuitry effectively creates a negative transconductance (or negative resistance) that cancels out the positive resistance associated with the losses in the resonators thereby sustaining the resonator's oscillations.
A 3-point oscillator, also known as a single-ended Pierce oscillator, is a stable active drive circuit which is combined with piezoelectric resonators for wired and wireless communication systems.
FIG. 1 shows a single-ended oscillator 100. The oscillator comprises a resonator 102, and a feedback gain stage 104. The gain stage consists of a feedback bias resistor 106 in combination with a transistor 108 for providing a negative transconductance to overcome losses in the resonator 102. The oscillator 100 also comprises load capacitors 110 which help cancel out the inductive behavior of the resonator 102 near parallel resonance frequencies (the self-resonance frequency of the resonator where it acts like an open circuit).
FIG. 2 shows another single-ended oscillator 200 that is similar to the single-ended oscillator 100 shown in FIG. 1, the difference being that the gain stage 204 uses a CMOS inverter logic gate 208.
A balanced oscillator, also known as a differential oscillator, is another type of oscillator that can be combined with a resonator to generate a signal. A differential oscillator produces differential signals. Differential signals are, when viewed together, a pair of complementary signals with better common-mode noise rejection and increased oscillation swing across the resonator as compared to the signal from a single-ended oscillator.
FIG. 3A shows a differential LC oscillator 300 arranged as a negative resistance active circuit. The oscillator 300 comprises a current source 302, an LC resonator 304, and a pair of cross-coupled PMOS transistors 306 connected in parallel with a pair of cross-coupled NMOS transistors 308 so as to create two back-to-back inverters 310. The back-to-back inverters 310 provide negative resistance to the LC resonator 304 for oscillation. Negative resistance replaces the energy lost by the resonator 304, thereby maintaining its vibration and, accordingly, the oscillations in the LC oscillator 300. To commence oscillation of the oscillator 300, Gate terminals of all transistors 306, 308 are initially shorted together through the direct current (DC) path in the inductor. Any electrical noise present in the oscillator 300 will then force the terminals of the LC resonator 304 to toggle and oscillate. Only the resonance frequency, however, will be selected by the resonator and amplified by the transistors in the loop of the oscillator 300. The LC resonator 304 cannot easily be substituted with a piezoelectric resonator, however, to improve the oscillator's 300 quality factor.
FIG. 3B shows a modified Butterworth-Van-Dyke (mBVD) circuit model 350 of a piezoelectric resonator. The circuit model 350 is essentially a lumped resistance (R), inductance (L), and capacitance (C) equivalent circuit. If the LC resonator 304 is replaced with the circuit model 350 of the piezoelectric resonator, and a direct current applied to thereto, the resonator behaves like an open circuit. The gate voltages of the transistors 306, 308 in the oscillator 300 latch to complementary high and low voltage levels due to the infinite-gain direct current response, instead of oscillating between two voltages.
To help prevent latching when using a piezoelectric resonator in a differential oscillator, a shunt resistor with a low impedance is used to provide a bias path. The shunt resistor needs to have a small resistance, however, reducing the quality factor of the resonator and degrading the phase noise of the oscillation signal.
Another option to help prevent latching in a differential oscillator when using a piezoelectric resonator is to insert capacitors between two “half circuits” to cut off the low-frequency and DC gain.
FIG. 4 shows a differential oscillator 400 with a piezoelectric resonator 402, cross-coupled transistors 404 realizing negative resistance (comprising two NMOS transistors 406), two DC current sources 408, resistors 410 and a capacitor 412. The capacitor 412 is positioned between two “half circuits” to reduce low-frequency gain and DC gain, and inhibits the gate voltages of the transistors 406 from latching to the complementary high and low voltages. This approach, however, may result in spurious oscillations and phase noise in the electronic signal. Namely, charging and discharging the capacitor 404 creates a periodic modulation that either appears as sidebands in the oscillator's electronic signal, or overcomes the electronic signal creating unwanted “relaxation oscillations”. The value of the capacitor 404, accordingly, needs to be sufficiently small to avoid unwanted modulations in the signal. A small capacitor, however, will cause degradation in the main oscillation swing, and phase noise degradation in the signal. This oscillator 400 is also difficult to implement since the common-mode voltage (the voltage about which the electronic signal oscillates) at the terminals of the resonator 402 depend on the values of the current sources 408 and resistors 410. The current sources 408 and resistors 410, moreover, reduce the voltage headroom for oscillation and degrade the phase noise performance.
FIG. 5 shows a differential oscillator 500 similar to the differential oscillator 400 shown in FIG. 4. The differential oscillator 500 has two NMOS transistors 510 (one on each side of the transistors 506) between the capacitor 512 and ground GND. The NMOS transistors 510 provide negative feedback to stabilize the common-mode voltages at the terminals of the resonator 502. The current sources 508 and the NMOS transistors 510, however, also occupy voltage headroom thereby degrading the phase noise performance of the signal produced by the oscillator 500.